Leadless cardiac stimulation device employing distributed logic

ABSTRACT

Systems and methods involve an intrathoracic cardiac stimulation device operable to provide autonomous cardiac sensing and energy delivery. The cardiac stimulation device includes a housing configured for intrathoracic placement relative to a patient&#39;s heart. A fixation arrangement of the housing is configured to affix the housing at an implant location within cardiac tissue or cardiac vasculature. An electrode arrangement supported by the housing is configured to sense cardiac activity and deliver stimulation energy to the cardiac tissue or cardiac vasculature. Energy delivery circuitry in the housing is coupled to the electrode arrangement. Detection circuitry is provided in the housing and coupled to the electrode arrangement. Communications circuitry may optionally be supported by the housing. A controller in the housing coordinates delivery of energy to the cardiac tissue or cardiac vasculature in accordance with an energy delivery protocol appropriate for the implant location.

FIELD OF THE INVENTION

The present invention relates generally to implantable medical devicesand, more particularly, to leadless systems and methods for detectingcardiac activity and treating adverse cardiac conditions.

BACKGROUND OF THE INVENTION

The healthy heart produces regular, synchronized contractions. Rhythmiccontractions of the heart are normally initiated by the sinoatrial (SA)node, which are specialized cells located in the upper right atrium. TheSA node is the normal pacemaker of the heart, typically initiating60-100 heartbeats per minute. When the SA node is pacing the heartnormally, the heart is said to be in normal sinus rhythm.

If the heart's electrical activity becomes uncoordinated or irregular,the heart is denoted to be arrhythmic. Cardiac arrhythmia impairscardiac efficiency and may be a potential life-threatening event.Cardiac arrhythmias have a number of etiological sources, includingtissue damage due to myocardial infarction, infection, or degradation ofthe heart's ability to generate or synchronize the electrical impulsesthat coordinate contractions.

Bradycardia occurs when the heart rhythm is too slow. This condition maybe caused, for example, by impaired function of the SA node, denotedsick sinus syndrome, or by delayed propagation or blockage of theelectrical impulse between the atria and ventricles. Bradycardiaproduces a heart rate that is too slow to maintain adequate circulation.

When the heart rate is too rapid, the condition is denoted tachycardia.Tachycardia may have its origin in either the atria or the ventricles.Tachycardias occurring in the atria of the heart, for example, includeatrial fibrillation and atrial flutter. Both conditions arecharacterized by rapid contractions of the atria. Besides beinghemodynamically inefficient, the rapid contractions of the atria mayalso adversely affect the ventricular rate.

Ventricular tachycardia occurs, for example, when electrical activityarises in the ventricular myocardium at a rate more rapid than thenormal sinus rhythm. Ventricular tachycardia may quickly degenerate intoventricular fibrillation. Ventricular fibrillation is a conditiondenoted by extremely rapid, uncoordinated electrical activity within theventricular tissue. The rapid and erratic excitation of the ventriculartissue prevents synchronized contractions and impairs the heart'sability to effectively pump blood to the body, which is a fatalcondition unless the heart is returned to sinus rhythm within a fewminutes.

Implantable cardiac rhythm management systems have been used as aneffective treatment for patients with serious arrhythmias. These systemstypically include one or more leads and circuitry to sense signals fromone or more interior and/or exterior surfaces of the heart. Such systemsalso include circuitry for generating electrical pulses that are appliedto cardiac tissue at one or more interior and/or exterior surfaces ofthe heart. For example, leads extending into the patient's heart areconnected to electrodes that contact the myocardium for sensing theheart's electrical signals and for delivering pulses to the heart inaccordance with various therapies for treating the arrhythmias describedabove.

Such implantable cardiac rhythm management systems include pacemakersand implantable cardioverter/defibrillators. A typical ICD, for example,includes one or more endocardial leads to which at least onedefibrillation electrode is connected. Such ICDs are capable ofdelivering high-energy shocks to the heart, interrupting the ventriculartachyarrythmia or ventricular fibrillation, and allowing the heart toresume normal sinus rhythm. ICDs may also include pacing functionality.

SUMMARY OF THE INVENTION

The present invention is directed to systems and methods that providefor detection of cardiac activity using a distributed logic approach.Embodiments of the invention are also directed to methods and devicesthat further provide for energy delivery to cardiac tissue using adistributed logic approach. Embodiments of the invention are alsodirected to leadless cardiac sensing and/or energy delivery devices,including those that are implantable in, on, or about the heart.

According to embodiments of the present invention, an intrathoraciccardiac stimulation device is operable to provide autonomous cardiacsensing and energy delivery. The cardiac stimulation device includes ahousing configured for intrathoracic placement relative to a patient'sheart. A fixation arrangement is supported by the housing and configuredto affix the housing at an implant location within cardiac tissue orcardiac vasculature. An electrode arrangement is supported by thehousing and configured to sense cardiac activity and deliver stimulationenergy to the cardiac tissue or cardiac vasculature.

Energy delivery circuitry is provided in the housing and coupled to theelectrode arrangement. Detection circuitry is provided in the housingand coupled to the electrode arrangement. A controller is provided inthe housing and coupled to the energy delivery circuitry and detectioncircuitry. The controller coordinates delivery of energy to the cardiactissue or cardiac vasculature in accordance with an energy deliveryprotocol appropriate for the implant location. Optional communicationscircuitry may be supported by the housing to effect communicationsbetween plural intrathoracic cardiac stimulation devices and/or apatient-external system.

According to other embodiments, a cardiac system includes two or more ofsuch intrathoracic cardiac stimulation devices. Each of the cardiacstimulation devices is configured for intrathoracic placement relativeto the patient's heart. The controller of each cardiac stimulationdevice coordinates delivery of energy in accordance with an energydelivery protocol appropriate for the implant location at which theparticular cardiac stimulation device is affixed.

In accordance with further embodiments, an intrathoracic cardiacstimulation method involves sensing cardiac activity from each of aplurality of intrathoracic locations, and delivering, from the pluralityof intrathoracic locations, energy to cardiac tissue or cardiacvasculature responsive to the sensed cardiac activity. The methodfurther involves coordinating, at the plurality of intrathoraciclocations, the energy delivery in accordance with a distributed energydelivery protocol. In other embodiments, the method may further involveestablishing a communication link between the plurality of intrathoraciclocations, and coordinating, via the communication link, the energydelivery from each of the plurality of intrathoracic locations inaccordance with an energy delivery protocol.

The above summary of the present invention is not intended to describeeach embodiment or every implementation of the present invention.Advantages and attainments, together with a more complete understandingof the invention, will become apparent and appreciated by referring tothe following detailed description and claims taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a depiction of a heart, with an intrathoracic cardiacstimulation device implanted in a single chamber of the heart inaccordance with an embodiment of the present invention;

FIG. 1B is a depiction of a heart, with an intrathoracic cardiacstimulation device implanted in two chambers of the heart (e.g., dualchamber configuration) in accordance with an embodiment of the presentinvention;

FIG. 1C is a depiction of a heart, with an intrathoracic cardiacstimulation device implanted in three chambers of the heart (e.g.,bi-ventricular or resynchronization configurations) in accordance withan embodiment of the present invention;

FIG. 1D is a depiction of a heart, with an intrathoracic cardiacstimulation device implanted in each of the four chambers of the heart(e.g., bi-ventricular or resynchronization configurations) in accordancewith an embodiment of the present invention;

FIG. 2A depicts communication linkage between multiple intrathoraciccardiac stimulation devices that provides for coordinated energydelivery to the heart in accordance with an embodiment of the presentinvention;

FIG. 2B depicts bi-directional communication linkage between multipleintrathoracic cardiac stimulation devices that provides for coordinatedenergy delivery to the heart, FIG. 2B further showing communicationlinkage between each of the cardiac stimulation devices and apatient-external system in accordance with an embodiment of the presentinvention;

FIG. 2C depicts bi-directional communication linkage between multipleintrathoracic cardiac stimulation devices that provides for coordinatedenergy delivery to the heart, FIG. 2C further showing communicationlinkage between one of the cardiac stimulation devices and apatient-external system in accordance with an embodiment of the presentinvention;

FIG. 2D depicts uni-directional communication linkage between multipleintrathoracic cardiac stimulation devices that provides for coordinatedenergy delivery to the heart, FIG. 2D further showing communicationlinkage between one of the cardiac stimulation devices and apatient-external system in accordance with an embodiment of the presentinvention;

FIG. 3A illustrates an intrathoracic cardiac stimulation deviceconfigured for endocardial or epicardial implantation that providescardiac pacing therapy in accordance with an embodiment of the presentinvention;

FIG. 3B illustrates an intrathoracic cardiac stimulation deviceconfigured for transvenous implantation that provides cardiac pacingtherapy in accordance with an embodiment of the present invention;

FIG. 3C illustrates an intrathoracic cardiac stimulation deviceconfigured for endocardial or epicardial implantation that provides oneor both of cardiac defibrillation and cardioversion therapy, alone orwith pacing therapy, in accordance with an embodiment of the presentinvention;

FIG. 4 is a block diagram of electronic circuitry and components of anintrathoracic cardiac stimulation device in accordance with anembodiment of the present invention; and

FIG. 5 is a timing diagram that represents one of many possible timingscenarios for pacing multiple chambers of the heart using anintrathoracic cardiac stimulation system of the present invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail below. It is to be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the invention isintended to cover all modifications, equivalents, and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

In the following description of the illustrated embodiments, referencesare made to the accompanying drawings, which form a part hereof, and inwhich is shown by way of illustration, various embodiments in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized, and structural and functional changes maybe made without departing from the scope of the present invention.

An implanted device according to the present invention may include oneor more of the features, structures, methods, or combinations thereofdescribed hereinbelow. For example, a cardiac monitor or a cardiacstimulator may be implemented to include one or more of the advantageousfeatures and/or processes described below. It is intended that such amonitor, stimulator, or other implantable device need not include all ofthe features described herein, but may be implemented to includeselected features that provide for useful structures and/orfunctionality. Such a device may be implemented to provide a variety oftherapeutic or diagnostic functions.

Embodiments of the invention are directed to methods and devices thatprovide for detection of cardiac activity using a distributed logicapproach. Embodiments of the invention are also directed to methods anddevices that provide for energy delivery to cardiac tissue using adistributed logic approach. Embodiments of the invention are furtherdirected to methods and devices that provide for treatment of adversecardiac conditions, such as by provision of one or more of pacing,resynchronization, and defibrillation/cardioversion therapies, using adistributed logic approach. Embodiments of the invention are furtherdirected to leadless cardiac sensing and/or energy delivery devices,including those that are implantable in, on, or about the heart.

In accordance with various embodiments, a cardiac stimulation system ofthe present invention includes one or more cardiac stimulation devices,each of which provides for cardiac activity sensing and delivery ofenergy to cardiac tissue. Although such cardiac stimulation devices arepreferably implemented for intrathoracic placement relative to apatient's heart, it is understood that a cardiac stimulation system ofthe present invention may provide for coordinated cardiac sensing and/orenergy delivery using one or more cardiac stimulation devices placedoutside of the rib cage, such as at one or more subcutaneous,non-intrathoracic locations. Further, although it will be appreciatedthat a leadless intrathoracic cardiac stimulation system provides anumber of advantages over lead-based implementations, benefits providedby a distributed logic approach of the present invention may be realizedin cardiac stimulation devices and systems that employ one or moreleads.

A cardiac stimulation system of the present invention may be implementedto include features that overcome known weaknesses in conventionalapproaches. For example, the leads of a conventional pacemaker or ICDare considered by many of those skilled in the art to be a “weak link”of such systems. Conventional leads, for example, are known to be proneto fatigue failures. Such leads can be prone to insulation failures,typically due to abrasion. Leads can sometimes cause permanent occlusionof the veins, and can be difficult to extract during an explantprocedure.

According to embodiments of the present invention, an intrathoraciccardiac stimulation (ICS) system may be implemented as a leadlesssystem. An ICS system may further be implemented in a distributedmanner, such that multiple cardiac stimulation devices operatecooperatively to deliver cardiac stimulation therapy, such as a pacingor resynchronization therapy. By way of example, an ICS system may beimplemented to include two or more essentially autonomous butinteractive ICS devices implanted in two or more heart chambers. Each ofthe ICS devices are preferably leadless devices, and each includes apower source (e.g., long-life battery or an externally rechargeablebattery), control and other electronics, and electrodes configured toprovide for cardiac activity sensing and energy delivery in astand-alone manner, independent from other ICS devices of the system.Each of the ICS devices further includes communication circuitry thatfacilitates communication between individual ICS devices of the system.A communication link established between ICS devices is typically abi-directional communication link, but certain configurations mayprovide for uni-directional communication as between a coordinating ICSdevice and a subordinate ICS device.

In accordance with various embodiments, an ICS device of the presentinvention is operable to provide essentially autonomous cardiac sensingand energy delivery, yet operates in concert with other ICS devices inaccordance with an energy delivery protocol, such as a pacing,resynchronization, antitachycardia pacing, rate smoothing orregularization pacing, or defibrillation/cardioversion protocol orregimen. According to various apparatus embodiments, an ICS deviceincludes a housing configured for intrathoracic placement relative to apatient's heart. The ICS device incorporates a fixation arrangementsupported by the housing and configured to affix the housing at animplant location within cardiac tissue or cardiac vasculature. Anelectrode arrangement is supported by the housing and configured tosense cardiac activity and deliver stimulation energy to the cardiactissue or cardiac vasculature. The ICS device includes energy deliverycircuitry provided in the housing and coupled to the electrodearrangement. Energy produced by the energy delivery circuitry may bepacing energy, defibrillation/cardioversion energy, ornon-excitory/sub-threshold energy (e.g., for impedance testing,increasing cardiac contractility, reversal of remodeling), for example.

The ICS device further includes detection circuitry provided in thehousing and coupled to the electrode arrangement. Communicationscircuitry is supported by the housing. A controller is provided in thehousing and coupled to the communications circuitry, energy deliverycircuitry, and detection circuitry. The controller coordinates theoperation of the ICS device and further coordinates delivery of energyto the cardiac tissue or cardiac vasculature in accordance with anenergy delivery protocol appropriate for the implant location. In asystem deployment involving two or more ICS devices, the controllers ofthe ICS devices communicate by way of a communications link establishedvia each device's communications circuitry.

Although inter-device communication is considered advantageous, it isenvisioned in certain embodiments that some or all of the ICS devicescan have a simplified configuration, in which communications circuitryis excluded and the controllers of the simplified ICS devices areprogrammed to deliver energy to cardiac tissue in response topredetermined cardiac signals sensed by each of the simplified ICSdevices. In a variation of this embodiment, one of the ICS devices mayincorporate communications circuitry for effecting communication with apatient-external system, such as a programmer or interface to a networksserver (e.g., a server of an advanced patient management system). ThisICS device may incorporate memory for storing cardiac activity data andother data that can be transmitted to the external system.

The housing may be configured to assume a wide variety of shapes andgeometries, such as by having a generally egg shape configuration, byway of non-limiting example. The housing may include one or morerecesses or other arrangement configured to receive a distal portion ofan implantation drive member. The housing may also include anarrangement, such as a locking arrangement, that couples to a wire orother explant member that facilitates extraction of the ICS device. Inthe case of an intravenous configuration, the housing may include one ormore channels configured to facilitate perfusion of blood through oraround the housing. For example, the housing may include a single ormultiple bores through which blood may pass. The housing may includechannels or flutes to facilitate blood perfusion.

The electrode arrangement of the ICS device may be configured as anepicardial, endocardial, or intravenous electrode arrangement. In oneconfiguration, the electrode arrangement may include one or more annularelectrodes supported circumferentially about all or a portion of thehousing. The electrode arrangement may incorporate an annular electrodesupported circumferentially about at least a portion of the housing anda helical electrode extending from the housing. The electrodearrangement is preferably configured as a bipolar electrode arrangement.

The fixation arrangement of the ICS device may include one or more of ahelical screw, tines, or other acute fixation arrangement. The fixationarrangement may also incorporate one or more chronic fixationarrangements that promote ingrowth or adhesion with cardiac tissue. Thefixation arrangement of the ICS device may define an anode or cathode ofthe electrode arrangement, such as in the case of a helical fixationelectrode. In other configurations, the fixation arrangement may beelectrically isolated from the electrode arrangement. In the case of atransvenous implementation, the fixation arrangement may include one ormore fixation members configured to extend outwardly from the housingand engage an inner surface of a cardiac vessel. Such fixation membersmay be formed from a suitable polymeric material.

An ICS device may be configured to provide only cardiac activitysensing, but finds particular usefulness when configured to provide bothcardiac activity sensing and energy delivery capabilities. One, two, ormore ICS devices may be configured to cooperatively implement a cardiacrhythm management regimen, such as a pacing regime, wherein individualICS devices implement a portion of the cardiac rhythm managementregimen. By way of example, a first ICS device may be implanted in apatient's right atrium, and a second ICS device may be implanted in thepatient's right ventricle. A cardiac rhythm management regimen may beimplemented by the first and second ICS devices operating cooperatively,such that the first and second ICS devices respectively deliver pacingpulses to the right atrium and ventricle at appropriate times asdictated by the cardiac rhythm management regimen. As such, each ICSdevice performs sensing and energy delivery operations appropriate forthe device's specific implant location (right atrium or right ventriclein this illustrative example), and the coordinated sensing and energydelivery operations of all ICS devices implement a given cardiac rhythmmanagement regimen.

Accordingly, the energy delivery protocol implemented by an ICS devicemay be a pacing protocol appropriate for a specific chamber of theheart, a multiple chamber pacing protocol, such as a resynchronizationpacing protocol, or a multisite pacing protocol, whereby at least twoICS devices are implanted in the same heart chamber. The energy deliveryprotocol implemented by one or more ICS devices may be a defibrillationor cardioversion protocol. The energy delivery protocol implemented byone or more ICS devices may also include delivery of stimulation energyinsufficient to effect capture, such as for impedance testing orimproving cardiac contractility.

The communications circuitry provided in a given ICS device is typicallyconfigured to facilitate communication between the ICS device and one ormore other ICS devices. The communications circuitry of one or more ICSdevices is typically configured to effect communication with apatient-external system, such as a patient-external receiver ortransceiver, programmer, or communications interface of a patientmanagement system, for example. As discussed previously, thecommunications link between ICS devices is typically bi-directional, butmay be uni-directional as between particular ICS devices. Thecommunication link between one or more ICS devices and apatient-external system is typically bi-directional, but may beuni-directional in particular applications.

An ICS system (i.e., one or more ICS devices) may be configured to sensecardiac activity and detect adverse cardiac events or conditions, suchas one or more of cardiac arrhythmia (e.g., bradycardia, tachycardia,fibrillation), post-shock asystole, and poor cardiac output, forexample. An ICS system of the present invention may be configured todeliver one or more therapies to treat detected cardiac events orconditions, including tachycardia prevention therapy (e.g.,antitachycardia pacing therapy, cardioversion therapy, defibrillationtherapy), bradycardia prevention therapy, and an asystole preventiontherapy. In response to detecting an adverse cardiac event or conditionrequiring treatment, the ICS system determines which of severalavailable therapies is appropriate to treat the subject event orcondition and delivers an appropriate therapy.

Turning now to FIGS. 1A-1D, there is illustrated four configurations ofan ICS system in accordance with embodiments of the present invention.FIG. 1A is a depiction of a heart 100, with an intrathoracic cardiacstimulation device 102 implanted in the right atrium (RA) of a patient'sheart. FIG. 1A illustrates a single chamber implementation of a leadlessICS system which can be configured to provide one or both of cardiacactivity monitoring and energy delivery. The ICS device 102 shown inFIG. 1A may provide sensing and energy delivery functionality similar tothat offered by conventional single chamber lead-based pacemakers orICDs. Unlike such conventional devices, provision of ICS device 102eliminates the need for lead implantation and associated long termreliability concerns, and further eliminates the need for a subcutaneouspocket incision for accommodating a convention can/housing. Implantationof ICS device 102 may be accomplished using a variety of techniques,including use of a screwdriver stylet for endocardial and epicardialimplantation, for example. Explanting of ICS device 102 may, forexample, be accomplished using a wire that engages a locking mechanismprovided on the housing of the ICS device 102.

FIG. 1B shows an ICS system implementation in which an ICS device 102 isimplanted in each of the right atrium and right ventricle (RV) of apatient's heart. FIG. 1B illustrates a dual chamber implementation of aleadless ICS system which can be configured to provide one or both ofcardiac activity monitoring and energy delivery. The ICS devices 102shown in FIG. 1B may provide sensing and energy delivery functionalitysimilar to that offered by conventional dual chamber lead-basedpacemakers or ICDs. In one approach, each of the ICS devices 102 issubstantially the same in terms of construction, electronics, andfunctionality, which allows for a reduction in system complexity (e.g.,reduced programming and interfacing complexity) and cost. In anotherapproach, one of the ICS devices 102 is constructed as a coordinatingICS device and the other ICS device 102 is constructed as a subordinateICS device. As such, the ICS devices 102 may differ substantially fromone another in terms of construction, electronics, and functionality,examples of which will be discussed hereinbelow.

FIG. 1C shows an ICS system implementation in which an ICS device 102 isimplanted in each of the right atrium, right ventricle, and leftventricle (LV) of a patient's heart. FIG. 1D shows an ICS systemimplementation in which an ICS device 102 is implanted in each of theright atrium, right ventricle, left ventricle, and left atrium (LA) of apatient's heart. FIGS. 1C and 1D are intended to illustrate multiplechamber implementations of a leadless ICS system which can be configuredto provide one or both of cardiac activity monitoring and energydelivery. By way of example, the ICS systems shown in FIGS. 1C and 1Dmay be configured to provide bi-ventricular pacing therapies. By way offurther example, the ICS systems shown in FIGS. 1C and 1D may beconfigured to provide resynchronization pacing therapies.

The ICS devices 102 shown in FIGS. 1C and 1D may provide sensing andenergy delivery functionality similar to that offered by conventionalmultiple chamber lead-based pacemakers or ICDs. As in the case of theICS systems depicted in FIG. 1B, each of the ICS devices 102 shown inFIGS. 1C and 1D may be substantially the same in terms of construction,electronics, and functionality. In other implementations, at least oneof the ICS devices 102 may be constructed as a coordinating ICS deviceand the other ICS devices 102 may be constructed as subordinate ICSdevices.

Multiple chamber embodiments of an ICS system described herein may beused in connection with congestive heart failure (CHF) monitoring,diagnosis, and/or therapy. Methods, structures, and/or techniquesdescribed herein relating to CHF, such as those involving dual-chamberor bi-ventricular pacing/therapy, cardiac resynchronization therapy,cardiac function optimization, or other CHF related methodologies, canincorporate features of one or more of the following references:commonly owned U.S. patent application Ser. No. 10/270,035, filed Oct.11, 2002, entitled “Timing Cycles for Synchronized Multisite CardiacPacing;” and U.S. Pat. Nos. 6,411,848; 6,285,907; 4,928,688; 6,459,929;5,334,222; 6,026,320; 6,371,922; 6,597,951; 6,424,865; and 6,542,775,which are hereby incorporated herein by reference. Various ICS systemembodiments described herein may be used in connection with preferentialpacing/rate regularization therapies. Methods, structures, and/ortechniques described herein relating to such therapies, such as thoseinvolving single chamber, multi-chamber, multi-site pacing/therapy orother related methodologies, can incorporate features of one or more ofthe following references: commonly owned U.S. patent application Ser.No. 09/316,515, filed May 21, 1999, entitled “Method and Apparatus forTreating Irregular Ventricular Contractions Such As During AtrialArrhythmia;” and U.S. Pat. Nos. 6,353,759 and 6,351,669, which arehereby incorporated herein by reference.

FIGS. 2A-2D illustrate a number of methodologies that facilitatecommunication between individual ICS devices and between ICS devices andpatient-external systems. FIG. 2A illustrates four chambers of a heart100. An ICS device 102 is shown implanted in three of the four heartchambers; the right atrium, right ventricle, and left ventricle. In oneembodiment, each of the ICS devices 102 is equipped with communicationscircuitry that facilitates bi-directional communication between therespective ICS devices 102. In one illustrative example, each ICS device102 senses for a particular cardiac event (e.g., an intrinsic event) anddecides when to deliver cardiac stimulation energy based on theoperative cardiac rhythm management regimen, specific location of theICS device 102, and activity of other ICS devices 102. A timing diagramassociated with one of many possible pacing regimens is illustrated inFIG. 5, for example.

FIG. 2B illustrates another embodiment involving inter-device 102communications and communication between ICS devices 102 and apatient-external system 200. In FIG. 2B, a bi-directional communicationlink is established between an ICS device 102 implanted in a patient'sright atrium and an ICS device 102 implanted in the patient's rightventricle. The communication link established between two or more ICSdevices 102 may be a conventional radio frequency (RF) link that istraditionally used to effect communications between an implantable pulsegenerator and an external programmer as is known in the art. Thecommunication link between ICS devices 102 may be implemented using ashort-range wireless communication interface in each of the ICS devices102, such as an interface conforming to a known communications standard,such as a Bluetooth standard, IEEE 802 standards (e.g., IEEE 802.11), orother public or proprietary wireless protocol. Other usefulcommunication techniques and circuitry, aspects of which may beimplemented by an ICS device of the present invention, are disclosed inU.S. Pat. No. 6,141,588, which is hereby incorporated herein byreference.

A non-electromagnetic communication link may also be employed tofacilitate inter-device communication, such as a link that utilizespressure waves encoded with data. One such approach employspiezoelectric transducers that generate and detect high-speed pressurewaves, aspects of which are disclosed in U.S. Pat. No. 5,411,535, whichis hereby incorporated herein by reference. It is noted thatcommunications circuitry disposed in one or more ICS devices 102 mayalso facilitate unidirectional or bidirectional communication with oneor more external, cutaneous, subcutaneous, or otherwise implantablephysiologic or non-physiologic sensors (e.g., blood oxygen sensors,transthoracic impedance sensors, blood volume sensors, acoustic sensorsand/or pressure transducers, and accelerometers). Information acquiredby such sensors may be used to adjust a pacing or other energy deliverytherapy deliverable be an ICS system.

Another mode of effecting a communication link involves use oflow-voltage pulses encoded with data. The voltage and/or frequency(assuming a non-DC signal) is selected so as not to interfere withintrinsic or device-assisted pacing functions. Typically, thelow-voltage encoded pulses are conducted via cardiac tissue and do notcause a reaction in the tissue.

In the embodiment shown in FIG. 2B, the communication interface providedin each ICS device 102 facilitates bi-directional communication betweeneach ICS device 102 and a patent-external system 200. This communicationlink may be a conventional RF link or short-range wireless communicationlink as discussed above. Although a bi-directional communication linkbetween an ICS device 102 and a patient-external system 200 isconsidered desirable, it is contemplated that certain patient-externalsystems 200 may require only a unidirectional communication link, as inthe case of a patient activator that triggers a diagnostic ortherapeutic function of the ICS system.

The patient-external system 200 may be an external programmer thatincorporates a communications interface to provide for reception andtransmission of signals and data between the external programmer andcommunications circuitry of the ICS devices 102. In this manner,programming commands and data may be transferred between the ICS devices102 and the external programmer during and after implant. Using aprogrammer, a physician is able to set or modify various parameters usedby the ICS devices 102. For example, a physician may set or modifyparameters affecting sensing, detection, pacing, and defibrillationfunctions of the ICS devices 102, including pacing andcardioversion/defibrillation therapy modes. The patient-external system200 may take other forms, such as a portable or bed-side communicationstation, patient-carried/worn communication station, patient managementserver-based system, or other communications or diagnostic system.

FIGS. 2C and 2D illustrate additional embodiments involving inter-device102 communications and communication between ICS devices 102 and apatient-external system 200. In FIGS. 2C and 2D, the ICS devices 102 areof differing configuration. In particular, ICS device 102A isimplemented as a coordinating ICS device and ICS device 102B isimplemented as a subordinate ICS device. As shown, ICS device 102Aincorporates communication circuitry that facilitates communication withthe patient-external system 200, while ICS device 102B excludes thiscommunication capability. A bi-directional communication link betweenICS devices 102A and 102B provides for inter-device communications.External access to the subordinate ICS device 102B by thepatient-external system 200 may be effected by way of the bi-directionalcommunication link between the coordinating ICS device 102A and thepatient-external system 200 and the bi-directional communication linkbetween the coordinating ICS device 102A and the subordinate ICS device102B. As such, the communications facility of the coordinating ICSdevice 102A may be used as a communications router, in that access toone or more subordinate ICS devices 102B may be achieved via thecoordinating ICS device 102A. It is noted that an ICS system may includeone or more coordinating ICS devices 102A and one or more subordinateICS devices 102B.

In FIG. 2D, ICS device 102A incorporates communication circuitry thatfacilitates bi-directional communication with the patient-externalsystem 200, while ICS device 102B excludes this communicationcapability. A unidirectional communication link is established betweenthe coordinating ICS device 102A and the patient-external system 200 inthis embodiment. Although a bi-directional communications link betweenICS devices 102A and 102B is generally desirable, certain ICS systemimplementations may allow for a reduced level of inter-devicecommunication, thus allowing for a more simplified inter-devicecommunication strategy. External access to the subordinate ICS device102B by the patient-external system 200 may be effected via thebi-directional communication link between the coordinating ICS device102A and the patient-external system 200 and the uni-directionalcommunication link between the coordinating ICS device 102A and thesubordinate ICS device 102B.

FIG. 3A illustrates an ICS device 300A configured for endocardial orepicardial implantation that provides cardiac pacing therapy inaccordance with an embodiment of the present invention. In thisillustrative embodiment, the ICS device 300A includes a housing 302 thatsupports an electrode arrangement and an antenna arrangement. Theelectrode arrangement according to this embodiment includes an annularelectrode 306, operative as an anode of the electrode arrangement, and afixation screw 304, operative as a cathode of the electrode arrangement.An antenna 308 (e.g., an RF antenna) is configured as a ring supportedon the housing 302. It is noted that the annular electrode 306 andannular antenna 308 may take on other shapes and geometries depending onthe shape or geometry of the housing 302.

It is noted that the electrode arrangement and/or the housing mayincorporate a mechanism for delivering or eluting a steroid or otherpharmacological agent, typically for purposes of reducing voltagethresholds. Such mechanisms may include a coating or sleeve disposed onthe electrode that contains a steroid or other pharmacological agent.Other systems and materials useful for drug delivery in accordance withthe present invention are further described in commonly owned U.S. Pat.Nos. 4,819,662 and 6,361,780, which are hereby incorporated herein byreference. It is further noted that the electrode arrangement mayinclude a fractal electrode coating, such as one that comprises iridiumoxide, as is known in the art.

A recess 307 is provided on the housing 302 and configured to receive adrive member, such as a stylet. For example, the recess 307 may be akey-hole that receives a drive member having an appropriate size andshape. The recess 307 or other arrangement may be configured to couplewith a wire or other member for purposes of extracting the ICS device300A from the heart. This coupling arrangement may include a lockingmechanism that engages an extraction member during an explant procedure.

FIG. 3B illustrates an ICS device 300B configured for transvenousimplantation that provides cardiac pacing therapy in accordance with anembodiment of the present invention. In this illustrative embodiment,the ICS device 300B includes a pair of annular electrodes 306A, 306Bthat respectively define an anode and a cathode of the ICS device'selectrode arrangement. An annular antenna 308 is provided on the housing302. The ICS device 300B also includes a fixation arrangement thatincludes one or more fixation members 312 configured to extend outwardlyfrom the housing 302 and engage an inner surface of a cardiac vessel. Inthe embodiment of FIG. 3B, two such fixation members 312 are shown. Thefixation members 312 may be polymeric rings of a diameter greater thanthat of the housing 302. The polymeric rings 312 are sufficientlyflexible to allow for some deformation during implantation, yet havesufficient strength to resist deflection in response to forcesassociated with blood flow once situated at an implant site.

The fixation members 312 may be perforated to allow for blood perfusion.The housing 302 may also include one or more bores 314 to facilitateperfusion of blood through the housing 302. The housing 302 mayalternatively, or additionally, include channels or flutes that allowfor perfusion of blood around the housing 302. The housing 302 mayincorporate an occlusion arrangement, such as one or more inflatableocclusion balloons or flutes, that can be actuated during implantation,but retracted/deactivated after implantation. Such an occlusionmechanism may be used in connection with an implant procedure within acardiac vessel, whereby a radio-opaque dye is injected into the vesselfor purposes of locating the housing 302 during implant as is known inthe art.

The central bore 314 shown in FIG. 3B may include a recessed or shapedportion configured to receive a drive member for purposes ofimplantation. A coupling arrangement may be built into the central bore314 or elsewhere on the housing 302 and configured to receive a wire orother member for purposes of extracting the ICS device 300B from theheart. As in the previous embodiment, this coupling arrangement mayinclude a locking mechanism that engages an extraction member during anexplant procedure. A transvenous stimulation apparatus and method,aspects of which may be implemented by an ICS device of the presentinvention, are disclosed in U.S. Pat. No. 6,445,953, which is herebyincorporated herein by reference.

FIG. 3C illustrates an intrathoracic cardiac stimulation deviceconfigured for endocardial or epicardial implantation that provides oneor both of cardiac defibrillation and cardioversion therapy, alone orwith pacing therapy, in accordance with an embodiment of the presentinvention. In this embodiment, the housing 302 includes a fixation screw304 and an annular antenna 308. The housing 302 supports an annularelectrode 306 and an elongated electrode 310. The annular electrode 306and elongated electrode 310 operate as a defibrillation electrode pairfor delivery of high energy cardioversion or defibrillation waveforms(e.g., mono-phasic, bi-phasic, or tri-phasic waveforms). The annularelectrode 306 and elongated electrode 310 also operate as a senseelectrode pair. The housing 302 may include a second annular electrode306 that allows for bi-polar cardiac activity sensing and/or pacing, inwhich case the elongated electrode 310 may be used exclusively fordelivering cardioversion or defibrillation therapy. A stimulationapparatus and method, aspects of which may be implemented by an ICSdevice of the present invention, are disclosed in U.S. Pat. No.5,405,367, which is hereby incorporated herein by reference.

FIG. 4 is a block diagram of electronic circuitry and components of anICS device 300 in accordance with an embodiment of the presentinvention. It is understood that the components and functionalitydepicted in FIG. 4 and described herein may be implemented in hardware,software, or a combination of hardware and software. It is furtherunderstood that the components and functionality depicted as separate ordiscrete blocks/elements in FIG. 4 may be implemented in combinationwith other components and functionality, and that the depiction of suchcomponents and functionality in individual or integral form is forpurposes of clarity of explanation, and not of limitation.

As shown in FIG. 4, the ICS device 300 includes a housing 302 thatsupports a fixation arrangement 414, an electrode arrangement 410, andan antenna 413. Disposed within the housing 302 are a number ofcomponents, including a controller 402, memory 403, detection circuitry404, energy delivery circuitry 406, and communications circuitry 412.The housing 302 also includes a power source 408, which may be along-life electrochemical battery, a fuel cell, a rechargeable battery,or other energy source.

In one configuration, the power source 408 includes a rechargeablebattery. According to this configuration, charging circuitry is coupledto the power source 408 to facilitate repeated non-invasive charging ofthe power source 408 from an external source. The communicationscircuitry 412, or separate receiver circuitry, is configured to receiveRF energy transmitted by an external RF energy transmitter. The ICSdevice 300 may, in addition to a rechargeable power source, include anon-rechargeable battery. It is understood that a rechargeable powersource need not be used, in which case a long-life non-rechargeablebattery is employed. An ICS device 300 may incorporate powertransmission and other control features disclosed in U.S. Pat. No.5,814,089, which is hereby incorporated herein by reference.

Communications circuitry 412 is disposed within the housing 302 andfacilitates communication between the ICS device 300 and an externalcommunication device 200, such as a portable or bed-side communicationstation, patient-carried/worn communication station, or externalprogrammer, for example. The communications circuitry 412 alsofacilitates unidirectional or bidirectional communication with one ormore other ICS devices 302 of an ICS system. The communicationscircuitry 412 may further facilitate unidirectional or bidirectionalcommunication with one or more external, cutaneous, subcutaneous, orotherwise implantable physiologic or non-physiologic sensors.

The controller 402 may includes a micro-processor coupled to appropriatememory 403 (volatile and non-volatile), it being understood that anylogic-based control architecture may be used. The controller 402 iscoupled to circuitry and components to sense, detect, and analyzeelectrical signals produced by the heart and deliver electricalstimulation energy to the heart under predetermined conditions to treatcardiac arrhythmias. In certain configurations, the controller 402 andassociated components also provide pacing therapy to the heart. Theelectrical energy delivered by the energy delivery circuitry 406 of theICS device 300 may be in the form of non-excitory energy, low energypacing pulses, or high-energy pulses for cardioversion ordefibrillation, for example.

Cardiac signals are sensed using the electrodes 410 provided on orsupported by the ICS device housing 302. The sensed cardiac signals arereceived by detection circuitry 404, which includes sense amplificationcircuitry and may also include filtering circuitry and ananalog-to-digital (A/D) converter. The sensed cardiac signals processedby the detection circuitry 404 may be received by noise reductioncircuitry, which may further reduce noise content of the sensed cardiacsignals.

Detection circuitry 404 may include a signal processor that coordinatesanalysis of the sensed cardiac signals and/or other sensor inputs todetect cardiac arrhythmias, such as, in particular, tachyarrhythmia.Rate based and/or morphological discrimination algorithms may beimplemented by the signal processor of the detection circuitry 404 todetect and verify the presence and severity of an arrhythmic episode.Exemplary arrhythmia detection and discrimination circuitry, structures,and techniques, aspects of which may be implemented by an ICS device 300of the present invention, are disclosed in commonly owned U.S. Pat. Nos.5,301,677 and 6,438,410, which are hereby incorporated herein byreference. Tiered or parallel arrhythmia discrimination algorithms mayalso be implemented using both rate-based and morphologic-basedapproaches. Further, a rate and pattern-based arrhythmia detection anddiscrimination approach may be employed to detect and/or verifyarrhythmic episodes, such as by use of the approaches disclosed in U.S.Pat. Nos. 6,487,443; 6,259,947; 6,141,581; 5,855,593; and 5,545,186,which are hereby incorporated herein by reference.

The detection circuitry 404 communicates cardiac signal information tothe controller 402. Memory circuitry 403 of the controller 402 containsparameters for operating in various sensing, pacing, and, if applicable,defibrillation modes, and stores data indicative of cardiac signalsreceived by the detection circuitry 404. The memory circuitry 403 mayalso be configured to store historical ECG and therapy data, which maybe used for various purposes and transmitted to an external receivingdevice as needed or desired.

In certain configurations, the ICS device 300 may include diagnosticscircuitry (not shown). The diagnostics circuitry typically receivesinput signals from the detection circuitry 404. The diagnosticscircuitry provides diagnostics data to the controller 402, it beingunderstood that the controller 402 may incorporate all or part of thediagnostics circuitry or its functionality. The controller 402 may storeand use information provided by the diagnostics circuitry for a varietyof diagnostics purposes. This diagnostic information may be stored, forexample, subsequent to a triggering event or at predetermined intervals,and may include system diagnostics, such as power source status, therapydelivery history, and/or patient diagnostics. The diagnostic informationmay take the form of electrical signals or other sensor data acquiredimmediately prior to therapy delivery.

An ICS device 300 may incorporate a cardiac pacing capability inaddition to, or to the exclusion of, cardioversion and/or defibrillationcapabilities. The ICS device 300 may include pacing therapy circuitrywithin the energy delivery circuitry 406, which is coupled to thecontroller 402 and the electrodes 410. Upon command, the pacing therapycircuitry delivers pacing pulses to the heart in accordance with aselected pacing therapy. Control signals, developed in accordance with apacing regimen by pacemaker circuitry within the controller 402, areinitiated and transmitted to the pacing therapy circuitry where pacingpulses are generated. A pacing regimen may be modified by the controller402.

According to a configuration that provides cardioversion anddefibrillation therapies, the controller 404 processes cardiac signaldata received from the detection circuitry 404 and initiates appropriatetachyarrhythmia therapies to terminate cardiac arrhythmic episodes andreturn the heart to normal sinus rhythm. The controller 402 is coupledto shock therapy circuitry of the energy delivery circuitry 406. Theshock therapy circuitry is coupled to electrodes 410. Upon command, theshock therapy circuitry delivers cardioversion and defibrillationstimulation energy to the heart in accordance with a selectedcardioversion or defibrillation therapy. In a less sophisticatedconfiguration, the shock therapy circuitry is controlled to deliverdefibrillation therapies, in contrast to a configuration that providesfor delivery of both cardioversion and defibrillation therapies.Exemplary ICD high energy delivery circuitry, structures andfunctionality, aspects of which may be incorporated in an ICS device 300of a type that may benefit from aspects of the present invention aredisclosed in commonly owned U.S. Pat. Nos. 5,372,606; 5,411,525;5,468,254; and 5,634,938, and in U.S. Pat. Nos. 5,133,353; 5,179,945;5,314,459; 5,318,597; 5,620,466; and 5,662,688, which are herebyincorporated herein by reference.

In particular configurations, systems and methods may perform functionstraditionally performed by pacemakers, such as providing various pacingtherapies as are known in the art, in addition tocardioversion/defibrillation therapies. Exemplary pacemaker circuitry,structures and functionality, aspects of which may be incorporated in anICS device of the present invention, are disclosed in commonly ownedU.S. Pat. Nos. 4,562,841; 5,284,136; 5,376,106; 5,036,849; 5,540,727;5,836,987; 6,044,298; and 6,055,454, which are hereby incorporatedherein by reference.

An ICS device in accordance with the present invention may implementdiagnostic and/or monitoring functions as well as provide cardiacstimulation therapy. Exemplary cardiac monitoring circuitry, structuresand functionality, aspects of which may be incorporated in an ICS deviceof the present invention, are disclosed in commonly owned U.S. Pat. Nos.5,313,953; 5,388,578; and 5,411,031, which are hereby incorporatedherein by reference in their respective entireties.

The components, functionality, and structural configurations describedwith reference to FIG. 4 are intended to provide an understanding ofvarious features and combination of features that may be incorporated inan ICS device 300. It is understood that a wide variety of ICS and otherimplantable cardiac monitoring and/or stimulation device configurationsare contemplated, ranging from relatively sophisticated to relativelysimple designs. As such, particular ICS or cardiac monitoring and/orstimulation device configurations may include particular features asdescribed herein, while other such device configurations may excludeparticular features described herein.

An ICS device of the present invention may be used within the structureof an advanced patient monitoring (APM) system. APM systems may allowphysicians to remotely and automatically monitor cardiac and respiratoryfunctions, as well as other patient conditions. In one example, an ICSdevice or system may be equipped with various telecommunications andinformation technologies that enable real-time data collection,diagnosis, and treatment of the patient. Various embodiments describedherein may be used in connection with advanced patient management.Methods, structures, and/or techniques described herein, which may beadapted to provide for remote patient/device monitoring, diagnosis,therapy, or other APM related methodologies, may incorporate features ofone or more of the following references: U.S. Pat. Nos. 6,221,011;6,270,457; 6,277,072; 6,280,380; 6,312,378; 6,336,903; 6,358,203;6,368,284; 6,398,728; and 6,440,066, which are hereby incorporatedherein by reference.

FIG. 5 is a timing diagram that represents one of many possible timingscenarios for pacing multiple chambers of the heart using a leadlessintrathoracic cardiac stimulation system of the present invention. FIG.5 shows several representative actions taken by multiple ICS devices inresponse to cardiac events sensed by the ICS devices. These actions areprovided to illustrate aspects of cooperative pacing by use of multipleICS devices. It is to be understood that a broad spectrum of pacingprotocols/regimens may be implemented by use of multiple ICS devices ofthe present invention, ranging from relatively simple (e.g., as shown inFIG. 5) to relatively complex (e.g., resynchronization pacing).

In various embodiments, a pacing regimen may be effected by amultiplicity of ICS devices, wherein each of the ICS devices isconfigured to respond to certain cardiac signals independent from otherICS devices. In such embodiments, the ICS devices need not includecommunications circuitry to effect communication as between ICS devicesfor purposes of coordinating a distributed pacing regimen. Rather, eachICS device is programmed to deliver pacing pulses in accordance with anenergy delivery protocol appropriate for its implant location. In thisconfiguration, pacing logic is distributed among the ICS devices in amanner appropriate for implementing an overall pacing regimen for thepatient. As discussed previously, it may be desirable that at least oneof the ICS devices be configured with communications circuitry to effectcommunication between the ICS device and an external system.

In other embodiments, a pacing regimen may be effected by a multiplicityof ICS devices, wherein each of the ICS devices is configured to respondto certain cardiac signals yet communicate with other ICS devices. Insuch embodiments, the ICS devices include communications circuitry toeffect communication as between ICS devices for purposes of coordinatinga distributed pacing regimen. In this configuration, pacing logic isdistributed among the ICS devices in a manner appropriate forimplementing an overall pacing regimen for the patient, andcommunication between ICS devices may enhance implementation of suchpacing regimen. Communication between ICS devices may provide forimplementation of more sophisticated pacing regimens, such asbi-ventricular or resynchronization pacing therapies.

The following illustrative example describes a coordinated pacingmethodology by use of multiple ICS devices that operate independentlyfrom one another, but together implement a dual chamber pacing regiment.In this illustrative example, it is assumed that the ICS devices do notcommunicate with one another to implement a dual chamber pacing regimen.As previously discussed one of the ICS may optionally incorporatecommunications circuitry to facilitated communications with apatent-external system.

According to this illustrative example, and with reference to the timingscenario of FIG. 5, it is assumed that a first ICS device is implantedin a patient's right ventricle and a second ICS device is implanted inthe patient's right atrium. During each cardiac cycle, each of the firstand second ICS devices “listens” (e.g., senses) for cardiac activityapplicable to the heart chamber in which the particular ICS device isimplanted. In response to detecting certain cardiac events, each of thefirst and second ICS device implements its particular pacing protocol.

The timing diagram of FIG. 5 shows three complete cardiac cycles (cc).The first cardiac cycle is initiated in response to the first ICS devicesensing depolarization (e.g., an R-wave) of the right ventricle, whichrestarts a ventricular escape interval (CCI or cardiac cycle interval).The second ICS device also senses the right ventricular depolarizationevent, which causes initiation of an atrial escape interval (AI oratrial interval).

In the first complete cardiac cycle shown in FIG. 5, it can be seen thatthe second ICS device delivers a pace to the right atrium in response toexpiration of the AI interval. The first ICS device senses an intrinsicRV depolarization prior to expiration of the ventricular escape interval(CCI), which inhibits delivery of a right ventricular pace by the firstICS device and restarts the escape interval timing of the first andsecond ICS devices (e.g., restarts the CCI and AI intervals).

In the second full cardiac cycle, which is initiated by a V-sense at theend of the first cardiac cycle, a second V-sense event is sensed by thefirst ICS device prior to the second ICS device sensing an atrialdepolarization. This scenario can occur in the case of a prematureventricular contraction (PVC), in which two consecutive ventriculardepolarization events occur in the absence of an intervening atrialdepolarization event. In this case, sensing of the V-sense event by thefirst ICS device results in inhibiting delivery of a V-pace by the firstICS device and an A-pace by the second ICS device, terminates thecurrent ventricular escape interval (CCI), and restarts the pace timingof the first and second ICS devices.

In the third full cardiac cycle, the first ICS device senses a V-senseevent and restarts the ventricular escape interval (CCI). The second ICSdevice also senses the V-sense event and restarts the atrial escapeinterval (AI). In this cardiac cycle, expiration of the atrial escapeinterval results in delivery of an A-pace by the second ICS device.Expiration of the ventricular escape interval results in delivery of aV-pace by the first ICS device. The next cardiac cycle is then initiatedupon delivery of the V-pace, which results is restarting of the atrialand ventricular escape intervals for the next cardiac cycle.

The following illustrative example describes a coordinated pacingmethodology by use of multiple ICS devices that are equipped withcommunications circuitry to effect communications there between.According to this illustrative example, and with continued reference tothe timing scenario of FIG. 5, it is assumed that a first ICS device isimplanted in a patient's right ventricle and a second ICS device isimplanted in the patient's right atrium. The first cardiac cycle isinitiated in response to the first ICS device sensing depolarization ofthe right ventricle, which restarts a ventricular escape interval (CCIor cardiac cycle interval). A signal is communicated from the first ICSdevice to the second ICS device causing the second ICS device to restartan atrial escape interval (AI or atrial interval).

In the first complete cardiac cycle shown in FIG. 5, the second ICSdevice delivers a pace to the right atrium in response to expiration ofthe AI interval. The first ICS device senses an intrinsic RVdepolarization prior to expiration of the ventricular escape interval(CCI), which inhibits delivery of a right ventricular pace by the firstICS device and restarts the escape interval timing of the first andsecond ICS devices. It is noted that, in a more complex pacing scenario,the second ICS device could transmit a signal to the first ICS device inresponse to delivery of the A-pace at the expiration of the AI interval.The first ICS device, in response, could initiate an AV interval thatcould be shorter or longer than the CCI interval, thus allowing foratrial tracking functionality (e.g., metabolic rate-based pace timing).

In the second full cardiac cycle, which is initiated by a V-sense at theend of the first cardiac cycle, a second V-sense event is sensed by thefirst ICS device prior to the second ICS device sensing an atrialdepolarization. In this case, sensing of the V-sense event by the firstICS device results in inhibiting delivery of a V-pace by the first ICSdevice and an A-pace by the second ICS device, terminates the currentventricular escape interval (CCI), and restarts the pace timing of thefirst and second ICS devices.

In the third full cardiac cycle, the first ICS device senses a V-senseevent, restarts the ventricular escape interval (CCI), and communicatesa signal to the second ICS device causing restarting of the atrialescape interval (AI). In this cardiac cycle, expiration of the atrialescape interval results in delivery of an A-pace by the second ICSdevice. Expiration of the ventricular escape interval results indelivery of a V-pace by the first ICS device. The next cardiac cycle isthen initiated upon delivery of the V-pace, which results is restartingof the atrial and ventricular escape intervals for the next cardiaccycle.

As was discussed previously, each of the ICS devices of a multiple ICSdevice system may be substantially the same or different in terms ofconstruction, components, and functionality. Within the context of thetiming scenario of FIG. 5, the first and second ICS devices may beessentially the same in construction, but obviously differ in terms oftheir sense/energy delivery timing protocol as appropriate for theirparticular heart chamber implant location. In one approach, nocommunication link exists between the two ICS devices. In anotherapproach, a communication link exists between the two ICS devices thatmay be uni-directional or bi-directional. In one configuration, forexample, each ICS device may communicate pacing event/timing signals toother ICS devices to implement a multi-device, distributed pacingprotocol or regimen.

In another approach, one of the ICS devices may coordinate cardiacpacing to a greater extent than other ICS devices of an ICS system. Inthe context of the pacing scenario depicted in FIG. 5, for example, thefirst ICS device implanted in the right ventricle may be considered acoordinating ICS device, in that cardiac pacing timing of the first andsecond ICS devices is registered with respect to right ventricularevents (i.e., ventricular depolarization events or lack thereof). In oneimplementation, the communication link between the first and second ICSdevices may be uni-directional, wherein atrial escape intervalrestarting is initiated by the second ICS device in response to an AIrestart signal received from the first ICS device. The second ICSdevice, which may be viewed as a subordinate ICS device in thisillustrative example, need not communicate timing related signals to thefirst ICS device to implement the relatively simplistic pacing timingshown in FIG. 5.

In accordance with a further approach, as discussed above, it may bedesirable to eliminate the communication link between the first andsecond ICS devices to implement a particular pacing protocol. In thecontext of the pacing timing diagram of FIG. 5, for example, the secondICS device located in the right atrium may be configured to be highlysensitive to the R-waves generated by ventricular depolarization events.In such a configuration, the second ICS device may be configured todetect R-waves and implement its pacing timing based on detectedR-waves. In this example, the first and second ICS devices operatecompletely autonomously, in that the two devices are not communicativelycoupled by a communication link, yet operate cooperatively to implementa given pacing regimen.

Various modifications and additions can be made to the embodimentsdiscussed hereinabove without departing from the scope of the presentinvention. Accordingly, the scope of the present invention should not belimited by the particular embodiments described above, but should bedefined only by the claims set forth below and equivalents thereof.

What is claimed is:
 1. A system of intrathoracic cardiac stimulationdevices operable to provide cardiac sensing and energy delivery,comprising: a plurality of autonomous intrathoracic cardiac stimulationdevices, each intrathoracic cardiac stimulation device comprising: ahousing configured for intrathoracic placement relative to a patient'sheart; a fixation arrangement supported by the housing and configured toaffix the housing at an implant location within cardiac tissue orcardiac vasculature; an electrode arrangement supported by the housingand configured to sense cardiac activity and deliver stimulation energyto the cardiac tissue or cardiac vasculature; energy delivery circuitryprovided in the housing and coupled to the electrode arrangement;detection circuitry provided in the housing and coupled to the electrodearrangement; and a controller provided in the housing and coupled to theenergy delivery circuitry and detection circuitry, the controllercoordinating delivery of energy to the cardiac tissue or cardiacvasculature in accordance with an energy delivery protocol appropriatefor the implant location such that the controller independentlycoordinates delivery of energy appropriate for the respective implantlocation at which the housing is affixed.
 2. The system of claim 1,wherein the electrode arrangement comprises an epicardial or endocardialelectrode arrangement.
 3. The system of claim 1, wherein the electrodearrangement comprises an intravenous electrode arrangement.
 4. Thesystem of claim 1, wherein the electrode arrangement comprises one ormore annular electrodes supported circumferentially about a portion ofthe housing.
 5. The system of claim 1, wherein the housing comprises oneor more recesses configured for one or both of receiving a distalportion of an implantation drive member and receiving an explant memberattachment mechanism.
 6. The system of claim 1, wherein the energydelivery protocol comprises a pacing protocol appropriate for a specificchamber of the heart.
 7. The system of claim 1, wherein the energydelivery protocol comprises a multiple chamber pacing protocol.
 8. Thesystem of claim 1, wherein the energy delivery protocol comprises adefibrillation or cardioversion protocol.
 9. The system of claim 1,further comprising communications circuitry coupled to the controller,wherein the controller coordinates communication with a patient-externalsystem via the communications circuitry.
 10. The system of claim 1,further comprising communications circuitry coupled to the controller,wherein the controller coordinates communication with one or more of theother intrathoracic cardiac stimulation devices via the communicationscircuitry.
 11. The system of claim 1, wherein each of the energydelivery protocols defines a unique portion of a cardiac rhythmmanagement regimen.
 12. The system of claim 1, wherein the controllersof the plurality of cardiac stimulation devices implement a cardiacpacing regimen via coordinated implementation of the respective energydelivery protocols.
 13. The system of claim 1, wherein the controllersof the plurality of cardiac stimulation devices implement a cardiacresynchronization pacing regimen via coordinated implementation of therespective energy delivery protocols.
 14. The system of claim 1, whereineach of the cardiac stimulation devices comprises communicationscircuitry coupled to the controller, wherein the controller coordinatescommunication with a patient-external system via the communicationscircuitry.
 15. The system of claim 14, wherein the communicationscircuitry of each of the plurality of cardiac stimulation devicesfacilitates bidirectional communications between the respectivecontrollers and the patient-external system.
 16. The system of claim 1,wherein: an implant location of a first cardiac stimulation device ofthe plurality of cardiac stimulation devices is associated with a firstheart chamber; and an implant location of a second cardiac stimulationdevice of the plurality of cardiac stimulation devices is associatedwith the first heart chamber.
 17. The system of claim 1, wherein: animplant location of a first cardiac stimulation device of the pluralityof cardiac stimulation devices is associated with a first heart chamber;an implant location of a second cardiac stimulation device of theplurality of cardiac stimulation devices is associated with a secondheart chamber; and an implant location of a third cardiac stimulationdevice of the plurality of cardiac stimulation devices is associatedwith a third heart chamber.
 18. The system of claim 1, wherein: animplant location of a first cardiac stimulation device of the pluralityof cardiac stimulation devices is associated with a first heart chamber;an implant location of a second cardiac stimulation device of theplurality of cardiac stimulation devices is associated with a secondheart chamber; an implant location of a third cardiac stimulation deviceof the plurality of cardiac stimulation devices is associated with athird heart chamber; and an implant location of a fourth cardiacstimulation device of the plurality of cardiac stimulation devices isassociated with a fourth heart chamber.